The present disclosure pertains to downhole oil-based mud (OBM) filtrate contamination monitoring (OCM) in real time. A known OCM approach utilizes an optical density mixing rule in which both optical density endpoints of a pure OBM filtrate and a pure native formation fluid are known. However, existing OCM is limited with respect to high gas-oil ratio (GOR) fluids, such as volatile oils and gas condensates, and for formation fluids with little to no optical density contrast relative to OBM filtrate.